The 19-year-old becomes the most expensive teenager in the history of football, eclipsing the £30m fee Van Gaal paid for Luke Shaw last year.

Monaco, who signed Martial from Lyon two years ago, rate the player highly, but the Ligue 1 club admitted United's offer, which could reportedly rise to £58m in total, was simply too good to turn down.

Monaco vice-president Vadim Vasilyev told the club's website: "We wished to keep him this season but Manchester United made an incredible offer that neither Anthony, nor AS Monaco, could refuse."

The forward, who scored 15 goals in 70 appearances for Monaco, has signed a four-year contract with the option of another year and will wear the number nine shirt for United.